1. Field of the Invention
This invention relates to a stent for keeping open tubular structures in the human body and to prevent restenosis by action eat.
2. Brief Description of the Prior Art
When treating the occurrence of stenoses in tubular structures such as vessels, the urinary tract and the like, or vascular aneurysms, stents or intraluminal tubes, i.e. generally tube-shaped supports made of metal and/or a polymer are implanted into the respective hollow viscus to keep the contracted structures open. The problem of using such implants is, however, that restenosis or an obstruction occurs soon after implantation, requiring another highly risky and costly surgical operation. When restenosis of cardiovascular stents occurs, an extensive bypass operation quite frequently is the only option.
Special catheters and microtools or laser tools are typically used to mechanically lay open the obstructive area of the stent under angiographic monitoring. Such regeneration, however, can be done twice at best. After that the stent support has to be replaced by a new implant.
Use of radioactive stents has been proposed to prevent the disadvantages mentioned above (U.S. Pat. No. 5,840,009, as re-embedding of endothelia or smooth muscle cells within the stent would not occur in the near range of radiation. However, exact dosage of radiation is difficult, and its cytotoxic effects are still uncertain.
Furthermore, stents have been described that are coated with anti-adhesion molecules (DE 197 13 240), fibrin/fibrinogen (U.S. Pat. No. 5,660,873), silicone (U.S. Pat. No. 5,330,500), or carbon (U.S. Pat. No. 5,163,958) or that come with a therapeutic delivery system (U.S. Pat. No. 5,439,446) to prevent restenosis.
Also known are stents made of a heat recoverable material (U.S. Pat. No. 5,197,978) that are connected to an electric heater, are introduced into a stenosed area of a hollow viscus and thermally dilated using a balloon catheter and can later be recovered to their original configuration. Finally publications (U.S. Pat. No. 5,178,618) describe expandable stents that can be heated to temperatures between 50xc2x0 C. and 100xc2x0 C. using external radiofrequency waves for ducting and stenosing tubular structures in the human body. Generation of heat in the stent material prevents proliferation of smooth muscle cells which are assumed to be responsible for restenosis of the stent and the resulting adverse consequences described at the outset.
Regeneration of electroconductive iron-containing restenosed stents in the body faces the setback that they only heat up at a relatively high field strength-frequency combination due to hysteresis and eddy-current losses that result in power absorption in the electroconductive tissue at the body surface and thus undesirable overheating of the peripheral adipose tissue and other uninvolved tissue. Regenerating metallic and non-metallic implants using heat has thus been unfeasible.
It is therefore the problem of this invention to provide a stent of the above type that, both in its metallic and non-metallic designs, can be selectively heated as desired to prevent restenosis or obstruction and to facilitate the stent""s growing into the respective hollow viscus.
This problem is solved according to the invention by providing a stent consisting of a metallic and/or non-metallic material and coated with nanoscale particles that comprise a paramagnetic core and a covering that can adsorb to the stent for position detection by MR tomography and for homogeneous and controlled heating and power absorption in an alternating magnetic field with a specific field strength and frequency suitable for clinical use.
The stent implant according to the invention, in metallic or non-metallic design, to the surface of which nanoscale particles are bonded in an even distribution pattern, makes it possible to set a controllable temperature that is restricted to the stent and slightly above the normal physiological temperature in a field strength and frequency range suitable for clinical use, thereby ensuring fast growing-in of the implant through enhancing the proliferation of new cells and regeneration of the restenosed stent at a temperature range from 50 to 60xc2x0 C. Due to their coating with nanoscale particles, both metallic and non-metallic designs of the implants are capable of high power absorption at field strengths below 10 kA/m and in a frequency range suitable for clinical use. Also, the stents heat up evenly. In addition, the provided coating allows position detection of the stent using MR tomography, no matter which implant material is used.
In a further improvement of the invention, said nanoscale particles consist of a core that contains, preferably consists of, ferromagnetic, ferrimagnetic, or preferably supramagnetic iron oxide and a covering made of at least one shell-adsorbing to the core. Said shell(s) comprise(s) reactive groups that can form cationic groups for permanent bonding of the outer shell to the surface of the stent. Said nanoscale particles are produced using known methods, such as the methods described in German laid-open patent publications nos. 195 15 820, 196 14 136, and 197 26 282.